1. Field of the Invention
The present invention disclosed in this specification relates to a touch panel including a touch sensor and a method for driving the touch panel. In particular, the present invention relates to a touch panel in which pixels each including a touch sensor are arranged in matrix and a method for driving the touch panel. Further, the present invention relates to a display device and an electronic device including the touch sensor.
2. Description of the Related Art
In recent years, a so-called touch panel, that is, a display device provided with a touch sensor, has attracted attention. Examples of the touch sensor includes a resistive touch sensor, a capacitive touch sensor, and an optical touch sensor which are different in operation principle. An object to be detected is touched to a display device, whereby data is input to the display device.
In an optical touch panel, a touch sensor which detects light is provided, and a display screen also serves as an input region. As an example of a device including such an optical touch sensor, a display device having an image capturing function by a contact area sensor, is given (for example, see Patent Document 1). In the optical touch panel, light is emitted from the touch panel. In the case where an object to be detected exists at a given position of the touch panel, light at the region where the object to be detected exists is blocked by the object to be detected, and part of light is reflected. A photo sensor (also referred to as a photoelectric conversion element) which can detect light is provided in a pixel of the touch panel; therefore, the photo sensor detects the reflected light, so that existence of the object to be detected in the region can be recognized.
In addition, it has been attempted to provide a personal authentication function or the like for an electronic device typified by a portable information terminal such as a mobile phone (for example, see Patent Document 2). A fingerprint, a face, a handprint, a palm print, a pattern of a hand vein, and the like are used for personal authentication. In the case where the personal authentication function is provided in a portion different from the display portion, the number of components is increased, and the weight or the price of the electronic device could possibly be increased. Patent Document 3 discloses a technique by which detection at a fingertip is performed in accordance with the brightness of outside light.